The invention relates to a portable, hand-operated working machine with a tool which is rotatable by an engine through a driving shaft, and a transmission comprising a centrifugal coupling, a driving belt, and a driving wheel with a groove for the driving belt fitting in said groove, which driving wheel comprises an outer and an inner half of the driving wheel, wherein the outer half of the driving wheel consists of an inner wall of a coupling drum being part of the centrifugal coupling, which drum is rotated by the driving shaft when the centrifugal coupling is in an operative state. The invention especially relates to a portable, hand-operated cutting machine or circular saw with a tool consisting of abutting or sawing blade, which e.g. may be a conventional sawing blade or an annular blade provided with diamond segments.
The invention also relates to a machine element which may be a part of the working machine. The invention has been developed in order to master problems with the machines of the above mentioned type, wherein the driving belt is of the V-belt type and the driving wheel has a wedge-shaped groove fitting the driving belt, but it is not limited to said application but may also be used with machines having other types of driving belts, e.g. so called poly-V-belts.
Driving belts of the V-type for working machines of the above mentioned type are subjects to heavy stresses. A problem is especially heavy heating of the driving belt, which implies that the driving belt often gets a very short duration time. The heating of the driving belt depends, partly on friction between the driving belt and the driving wheel, and the driving pulley, respectively, partly on heating of the coupling drum of the centrifugal coupling because of slipping of the coupling shoes in the drum, partly on the heat from the engine, when it is the question of an internal combustion engine. The heat which is developed in the coupling drum, being an integrated part of the driving wheel, is transmitted to the driving belt. According to prior art, different measures have been suggested to counteract the heating of the driving belt. It is for instance known to provide air openings in the belt cover, which is conventionally provided in the form of a driving belt cover at a distance from the driving belt and the centrifugal coupling, but none of said known devices for cooling the driving belt has given the desired result.
The object of the invention is to address the above mentioned problems. The characterising feature of the invention is that a fan is provided inside the driving belt, as viewed in the axial direction, coaxially to the driving shaft and arranged to rotate at least when the centrifugal coupling is in the operative state, which fan has fan blades extending at least partly radially beyond said groove for the driving belt, which fan blades are designed to suck cooling air in the axial direction towards the space outside said groove for the driving belt and towards the drum of the centrifugal coupling.
According to one embodiment, the inner driving wheel half is an integrated, inner portion of the fan, as viewed in the radial direction, while the fan blades form outer, periphery portions of the driving wheel half. Preferably, the two halves of the driving wheel are connected to each other, e.g. through a sleeve being journalled on the driving shaft, which is rotatable also when the driving wheel and the fan do not rotate or rotate at a lower speed, when the centrifugal coupling is in the non-operative condition or slips. In addition to the cooling air directly cooling the driving belt, or counteracting heating of the driving belt through cooling the coupling cover, said embodiment has the advantage that the manufacturing costs are reduced and that the assembly is simplified, as the coupling cover, the driving wheel, and the fan constitute an integrated unit.
According to another embodiment of the invention the fan is a separate unit provided inside the inner driving wheel half, as viewed in the axial direction, between the inner driving wheel half and the engine. Also in this case, the driving wheel is journalled on the driving shaft, which is rotatable when the driving wheel does not rotate or rotates at a lower speed, when the centrifugal coupling is not in the operative state or slips, while the fan, being a separate unit, may be fixedly mounted, i.e. not journalled on the driving shaft, to rotate together with driving shaft also when the engine is running but the centrifugal coupling is not in the operative state or slips. Said embodiment has the advantage that the fan cools the driving belt and the coupling drum also when the engine runs idle, and the driving shaft rotates at a lower speed as well as when the centrifugal coupling slips and develops heat in the coupling drum because of friction between the coupling shoe and the drum. The embodiment, where the fan is a separate unit provided inside the inner driving wheel half, does, however, not exclude that the fan also in this case is integrated with the driving wheel and forms an integrated unit, e.g. that both the driving wheel and the fan are mounted on a common sleeve being journalled on the driving shaft, which facilitates the assembly.
Further, according to an aspect of the invention, a driving belt cover is provided with a chamber being defined by a portion of the driving belt cover outside the driving belt and the centrifugal coupling, and by a shield wall extending around a portion of the centrifugal coupling and the driving wheel at least behind the centrifugal coupling and the driving wheel, but which chamber is open for the driving belt in the forward direction. According to this aspect of the invention, the centrifugal coupling and the driving wheel are provided in said chamber, and the fan is arranged to drive at least a partial flow of the cooling air into said chamber and therefrom further forwards inside the driving belt cover to further improve the cooling effect of the cooling air on the driving belt directly and indirectly by cooling the coupling drum. Preferably, the driving belt cover is made without any apertures, slots or the like in its wall with the purpose that the cooling air shall be driven into said chamber substantially in the axial direction towards the region outside the driving belt groove and towards the coupling drum and further from said chamber at least as a partial flow forwards along the driving belt inside the driving belt cover.
Further aspects and features of the invention will be apparent from the following description of the preferred embodiments.